Implantable cardiac devices are well known in the art. They may take the form of implantable defibrillators or cardioverters which treat accelerated rhythms of the heart such as fibrillation or implantable pacemakers which maintain the heart rate above a prescribed limit, such as, for example, to treat a bradycardia. Implantable cardiac devices are also known which incorporate both a pacemaker and a defibrillator.
A pacemaker may be considered as a pacing system. The pacing system is comprised of two major components. One component is a pulse generator which generates the pacing stimulation pulses and includes the electronic circuitry and the power cell or battery. The other component is the lead, or leads, having electrodes which electrically couple the pacemaker to the heart. A lead may provide both unipolar and bipolar pacing and/or sensing electrode configurations. In the unipolar configuration, the pacing stimulation pulses are applied or evoked responses are sensed between a single electrode carried by the lead, in electrical contact with the desired heart chamber, and the pulse generator case. The electrode serves as the cathode (negative pole) and the case serves as the anode (positive pole). In the bipolar configuration, the pacing stimulation pulses are applied or evoked responses are sensed between a pair of closely spaced electrodes carried by the lead, in electrical contact with the desired heart chamber, one electrode serving as the anode and the other electrode serving as the cathode.
Pacemakers deliver pacing pulses to the heart to cause the stimulated heart chamber to contract when the patient's own intrinsic rhythm fails. To this end, pacemakers include sensing circuits that sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial events (P waves) and intrinsic ventricular events (R waves). By monitoring such P waves and/or R waves, the pacemaker circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pacing pulses that force atrial and/or ventricular depolarizations at appropriate times in the cardiac cycle when required to help stabilize the electrical rhythm of the heart.
Implantable cardiac defibrillators (ICD's) are also well known in the art. These devices generally include an arrhythmia detector that detects accelerated arrhythmias, such as tachycardia or fibrillation. When such a tachyarrhythmia is detected, a pulse generator delivers electrical therapy to the patient's heart. A therapy for tachycardia may be anti-tachycardia pacing and a therapy for fibrillation may be a defibrillating shock. Such therapies for both atrial and ventricular tachyarrhythmias are well known.
Implantable cardiac devices find usefulness beyond the provision of the aforementioned therapies. For example, such device may be very useful in the collection data for various types of studies relating to the heart or for monitoring the disease state of a patient.
One parameter commonly important in cardiac data collection is cardiac interval. Cardiac interval determination requires reliable R wave detection. Unfortunately, reliable R wave detection is difficult under many commonly found conditions. Such conditions include varying baseline and changing morphology such as varying R wave amplitude and reduced R/T ratio. The use of fixed threshold R wave detection, commonly found in implantable cardiac devices, makes it difficult to reliably detect the R waves under the noted conditions.
It is very important to be able to precisely detect R waves and accurately determine the starting time of that complex in case of co-morbidity detection. For example, it is known that hypoglycemia can be detected based on monitoring changes in the QT interval observed within an electrocardiogram (ECG), as well as based on observation of dispersion of QT intervals within the ECG. Studies in diabetics have also shown that hypoglycemia can be detected based on observation of a significant lengthening of the QTc interval occurring during spontaneous nocturnal hypoglycemia. R wave detection error is incorporated in the QT interval error as well. All of this leads to poor quality of data for co-morbidity detection and reduces the specificity of the diagnostic. This is true for all co-morbidity detections and any therapy that depends on precise detection of R waves.